In atomic layer deposition (ALD), a film is deposited layer by layer by successive dosing and activation steps. ALD is used to generate conformal films on high aspect ratio structures. One of the drawbacks of ALD is that film deposition on the backside of the wafer is difficult to avoid because the film can be deposited through any gap accessing the wafer backside. Backside deposition is unwanted for a number of reasons, one of which is that excess film on the backside of the wafer is susceptible to flaking, e.g., during wafer transport. If flakes from the backside of the wafer come into contact with a wafer (either the same wafer or a different wafer), the wafer is contaminated and defects can result.
In capacitive coupled plasma chambers, a small gap is desired between the pedestal and the wafer. This gap induces impedance between the wafer and the pedestal that is sufficient to essentially drown out the impedance variations caused by variations of the pedestal surfaces on the micro scale. To create the small gap between the pedestal and the wafer, minimum contact area (MCA) features are used to create a level virtual offset place with supports that ensure a flat wafer plane. As noted above, the gap between the pedestal and the wafer provides access to the backside of the wafer and thereby facilitates film deposition on the backside of the wafer.
Pedestals used in conventional ALD and plasma-enhance chemical vapor deposition (PECVD) systems typically include multiple MCA features that are adjustable in height. The current pedestal design guidelines specify the use of enough MCA features to ensure that the support plane created by these features holds a wafer parallel to the pedestal. Indeed, a design is considered inadequate if the number of MCA features used allows for wafer deflection or sag. Thus, in some pedestal designs, more than 30 MCA features are used to support the wafer.
It is in this context that embodiments arise.